This invention relates generally to a process for the fractionation of a hydrocarbon mixture, for example, natural gas in a distillation column to produce a gaseous overhead fraction containing lighter hydrocarbons and sour gases, if present, and a liquid bottoms product containing heavier hydrocarbons.
It is conventional, for example, in the fractionation of a methane-containing gaseous mixture rich in C.sub.2+, to cool the crude gas against fractionation products and external refrigeration so as to condense the hydrocarbons. In this process, depending on temperature and pressure, the light components, such as, for example, N.sub.2, CH.sub.4, and C.sub.2 H.sub.6, preferentially remain in the gaseous phase. This gas/liquid mixture is then fractionated in a distillation column, i.e., a demethanizer, into a C.sub.2+ fraction as the bottoms product and a gaseous C.sub.1 fraction as overhead.
In order to improve the yield in C.sub.2, it is conventional to introduce at the head of the column a reflux stream obtained by condensation of the overhead product, the latter being comprised primarily of CH.sub.4 and C.sub.2 H.sub.6. However, obtaining this reflux condensate is very costly since very low temperatures are required for condensation.
Two processes, in principle, have been used for the formation of the reflux. According to one process, a partial stream of the overhead product is condensed by means of external refrigeration. This requires, depending on pressure and composition, a methane or C.sub.2 refrigeration cycle, optionally coupled with a C.sub.3 refrigeration cycle. High initial investment and operating costs are involved in the installation of a separate low temperature refrigeration cycle. Moreover, the internal recycle in the column is considerably increased. The yield of C.sub.2 is also impaired unless the CH.sub.4 is condensed at the head of the column. However, CH.sub.4 condensation is costly from an energy viewpoint especially if N.sub.2 is present in the feed gas. For further details, reference is invited to U.S. Pat. No. 4,582,517.
In accordance with the other process, the precooled gas, after separation of condensate, is subjected to engine expansion in an expansion turbine to produce a liquid fraction which is introduced as reflux to the head of the column. This expansion method has the drawback that the gas expanded in the turbine must be recompressed in certain instances before being discharged. In case of gases having a high content of C.sub.2+ hydrocarbons, such as, for example, gas associated with petroleum formations, the amount of gas remaining after partial condensation is relatively small. Thus, only a minor refrigerating value can be gained therewith by expansion of such small gaseous fractions. For further details of this expansion process, reference is invited to U.S. Pat. No. 4,278,457.
In both processes, very low temperatures of -50.degree. to -120.degree. C. are involved. This, in turn, requires thorough prepurification of the feedstock so as to remove congealable components such as water or CO.sub.2. Also, expensive low-temperature resistant materials of construction are required.